Controlling morphology at the nanometer scale has the potential to revolutionize technology through development of materials with exquisite control of mechanical, optical, electronic and structural properties. Moreover, recent research has led to a host of new fundamental scientific insights, including controlled nanoscale synthesis and processing of both organic (soft) and inorganic (hard) material and the development of nano-scale precursors for macroscopic materials and devices. A challenge, therefore, is to develop a hierarchical approach that can combine a variety of organic and inorganic building blocks, provide nanometer-scale structural control and simultaneously lead to macroscopic devices or materials in a practical and cost-effective way. Moreover, the approach must be flexible so that it can be readily extended to a variety of materials or properties without substantial revision of the entire process. These are demanding goals that require novel approaches and development of basic science.
Photolithography provides a means of generating structure, generally planar in nature, with a spatial resolution on the tenths of micron size scale, but this technique is limited to a small set of materials. Chemical synthesis can provide molecular resolution, but does not provide a robust and flexible method of independently controlling mechanical, structural, electronic and optical properties of a material.
Earlier attempts to synthesize organic/inorganic materials at fluid interfaces involved mesoporous silica or titania at the surface of a water droplet in oil or an oil droplet in water. These structures, however, do not contain well-defined features, nor can the composition of the oil or aqueous phases be altered without substantially changing the inorganic layer. Other approaches to interfacial self-assembly have used millimeter-sized objects with patterned hydrophobicity. This technique, however, has not been extended to the micro- or nano-scale.
Alternatively, electrostatic deposition of alternating layers of polymers or particles or polymerization on the surfaces of small particles or oil droplets provides a flexible route to encapsulation: the capsule is formed around a nanometer-to-micron-sized sphere or a crystal of a water-insoluble material. The capsule can then be swollen and filled from the surrounding phase. Such an approach works for a broad range of materials but the features, specifically pores, are restricted in size to a few nanometers or less and are not well defined in shape. In addition, encapsulation is limited only to water-insoluble objects or objects small enough to be inserted into the swollen pre-constructed capsule from the exterior.